The present invention pertains generally to implantable medical devices and, more particularly, to implantable medical devices which are capable of being implanted utilizing minimally-invasive delivery techniques. More particularly, the present invention relates to endoluminal grafts, stent-grafts and stent-graft-type devices that are implanted into anatomical passageways using minimally invasive delivery techniques. More specifically, the present invention comprises endoluminal grafts, stent-grafts and stent-graft-type devices that are fabricated entirely of biocompatible metals or of biocompatible materials which exhibit biological response and material characteristics substantially the same as biocompatible metals, such as for example composite materials.
Conventional endoluminal stents and stent-grafts are frequently used post-angioplasty in order to provide a structural support for a blood vessel and reduce the incidence of restenosis following percutaneous balloon angioplasty. A principal example of the present invention are endovascular stents which are introduced to a site of disease or trauma within the body's vasculature from an introductory location remote from the disease or trauma site using an introductory catheter, passed through the vasculature communicating between the remote introductory location and the disease or trauma site, and released from the introductory catheter at the disease or trauma site to maintain patency of the blood vessel at the site of disease or trauma. Stent-grafts are delivered and deployed under similar circumstances and are utilized to maintain patency of an anatomic passageway, for example, by reducing restenosis following angioplasty, or when used to exclude an aneurysm, such as in aortic aneurysm exclusion applications.
While the use of endoluminal stents has successfully decreased the rate of restenosis in angioplasty patients, it has been found that a significant restenosis rate continues to exist even with the use of endoluminal stents. It is generally believed that the post-stenting restenosis rate is due, in major part, to a failure of the endothelial layer to regrow over the stent and the incidence of smooth muscle cell-related neointimal growth on the luminal surfaces of the stent. Injury to the endothelium, the natural nonthrombogenic lining of the arterial lumen, is a significant factor contributing to restenosis at the situs of a stent. Endothelial loss exposes thrombogenic arterial wall proteins, which, along with the generally thrombogenic nature of many prosthetic materials, such as stainless steel, titanium, tantalum, Nitinol, etc. customarily used in manufacturing stents, initiates platelet deposition and activation of the coagulation cascade, which results in thrombus formation, ranging from partial covering of the luminal surface of the stent to an occlusive thrombus. Additionally, endothelial loss at the site of the stent has been implicated in the development of neointimal hyperplasia at the stent situs. Accordingly, rapid re-endothelialization of the arterial wall with concomitant endothelialization of the body fluid or blood contacting surfaces of the implanted device is considered critical for maintaining vasculature patency and preventing low-flow thrombosis.
At present, most endoluminal stents are manufactured of stainless steel, which is known to be thrombogenic. In order to reduce the thrombogenicity of the stainless steel and to maintain sufficient dimensional profiles for catheter delivery, most stents minimize the metal surface area that contacts blood, in order to minimize thrombus formation after implantation. Thus, in order to reduce the thrombogenic response to stent implantation, as well as reduce the formation of neointimal hyperplasia, it would be advantageous to increase the rate at which endothelial cells form endothelium proximal and distal to the stent situs, migrate onto and provide endothelial coverage of the luminal surface of the stent which is in contact with blood flow through the vasculature.
Stent-grafts are essentially endoluminal stents with a discrete covering on either or both of the luminal and abluminal surfaces of the stent that occludes the open spaces, or interstices, between adjacent structural members of the endoluminal stent. It is known in the art to fabricate stent-grafts by covering the stent with endogenous vein or a synthetic material, such as woven polyester known as DACRON, or with expanded polytetrafluoroethylene. Additionally, it is known in the art to cover the stent with a biological material, such as a xenograft or collagen. A primary purpose for covering stents with grafts is to reduce the thrombogenic effect of the stent material. However, conventional grafts are not a complete solution to enhancing the healing response of the devices.
Heretofore, the art has not provided 1) a graft fabricated of biocompatible metals or of biocompatible materials which exhibit in vivo biological and mechanical responses substantially the same as biocompatible metals (hereinafter referred to as “metal-like materials”); 2) a stent-graft device in which a structural component, or stent, and a graft component are each fabricated of metal or metal-like materials; and 3) a stent-graft-type device in which a structural support, such as a stent, defines openings which are subtended by a web, with both the stent and the web being formed as a single, integral, monolithic structure and fabricated of metals or of metal-like materials, this particular embodiment is hereinafter referred to as a “web-stent.”